1. Field of the Invention
The present invention relates to a variable focus lens that includes seven lenses and is provided in, for example, a projection display device, and a projection display device including the variable focus lens, and more particularly, to a small projection variable focus lens and a projection display device that enlarge and project light having image information from a light valve of, for example, a transmissive or reflective liquid crystal display device or a DMD (digital micro-mirror device) display device onto a screen.
2. Description of the Related Art
In recent years, projection display devices using light valves, such as liquid crystal display devices or DMD display devices, have come into widespread use. In particular, a projection display device has been widely used which uses three light valves corresponding to illumination light components of three primary colors, such as R, G, and B, to modulate the illumination light components, and combines the light components modulated by the three light valves using, for example, a color composition prism, and displays an image on a screen through a projection lens.
In many cases, the projection display device uses as a projection lens a variable focus lens (zoom lens) capable of changing the size of the image projected onto the screen. A telecentric variable focus lens including four lens groups or five lens groups is generally used as the projection variable focus lens. When a high performance or a high zoom ratio is required, a variable focus lens including six lens groups is also used.
In general, the variable focus lens includes a large number of lenses in order to achieve high aberration characteristics, ensure telecentricity, and prevent a reduction in contrast or the occurrence of color unevenness. However, when the number of lenses increases, manufacturing costs also increase. Therefore, it is necessary to construct a variable focus lens capable of achieving the objects with the minimum number of lenses.
In order to meet the requirements, Japanese Patent Nos. 4114515 and 3513514 (corresponding to U.S. Pat. No. 5,313,330) disclose projection variable focus lenses each including seven lenses.
However, in the projection zoom lens including seven lenses disclosed in Japanese Patent No. 4114515, four lens groups are moved when power varies. Therefore, a moving mechanism including, for example, a cam becomes complicated, and the weight of the projection zoom lens or the difficulty of manufacture increases, which results in an increase in manufacturing costs.
The projection zoom lens including seven lenses disclosed in Japanese Patent No. 3513514 has an F number of 6 and it is difficult to meet a demand for a fast lens system.